1. Field of the Invention
The present invention relates to a transformer, and more particularly, to a transformer adopting a structure of an integrated passive device (IPD) used in a complementary metal-oxide semiconductor (CMOS) power amplifier.
2. Description of the Related Art
In general, power amplifiers are used at Tx ports of mobile communication terminals, such as portable phones, to amplify the power of a Tx signal. The power amplifiers need to amplify a Tx signal to proper power. To control the output power of the power amplifier, a closed loop method or an open loop method may be used. As for the closed loop method, a transformer at an output port of a power amplifier detects a portion of an output signal, a Schottky diode converts the detected signal into DC current, and a comparator compares the DC current with a reference voltage. As for the open loop method, the power is regulated by sensing the voltage or current being applied to the power amplifier.
The closed loop method, which has been traditionally used, ensures precise power control but complicates circuit implementation and causes a deterioration in amplifier efficiency due to coupler loss. The open loop method allows simple circuit implementation and thus is being used in many cases, but fails to precisely regulate power.
Recently, circuit implementation in the closed loop method has been simplified due to the integration of components into an integrated circuit (IC). Also, with the performance enhancement of a control chip, the coupling value of directional couplers used for the closed loop method has been significantly lowered, thus greatly reducing coupler loss. Notably, the closed loop method that ensures precise power control is in current use in the Global System for Mobile communications (GSM) communication scheme where the ramping profile is considered important.
Studies are ongoing to effectively implement transformers controlling the output of power amplifiers. However, harmonic components are generated in an output signal when a transformer is used. Notably, when a transformer is configured as IPD employing a CMOS power amplifier, the characteristics of the harmonic components become complicated due to high output power.